1. Field of the Invention
The present invention relates to a heat-pipe system particularly to an externally pumped heat-pipe system of the two-phase fluid loop type for transport of heat by thermodynamic cycles involving the phase changes between evaporation and condensation, and a method for controlling a flow rate of a working fluid in a liquid pipe, so as to optimally control the fluid remaining in the capillary structure of an evaporator.
2. Description of the Prior Art
As a heat transport system such as a space station, a fluid loop is useful in view of the demand of large heat load capability for transport. In a single phase fluid loop, however, since heat is transported by use of radiated heat, a large amount of working fluid has to be circulated in order to maintain the temperature of a heating portion within a narrow temperature range. As a result, a large scale piping and pumping system is required so that it is not practical in view of weight as well as pumping power.
Accordingly, when large amounts of heat should be transported with less pumping power and the temperature of the heating portion should be maintained at a narrow temperature range, the two-phase fluid loop is considered useful, which transports heat by using latent heat due to changes in phase of the working fluid.
According to the system described above, since the latent heat is used in the two-phase fluid loop, the circulating amount of the working fluid can be reduced, thus enabling the piping and pumping system to be minituarized and light in weight as well as enabling use of a small driving power for circulation of the fluid, with sufficient practicality.
In the two-phase fluid loop, the circulating fluid is transported in liquid phase to heat radiating portions of electronics, where it is transformed into vapor phase by heat absorption. The vapor phase is transferred to a heat radiation portion, where heat is radiated and it is again returned to the liquid phase.
As one method for circulating such two-phase working fluids is described above, there are mainly two methods; one is a method of using, for instance, a mechanically pumped system and the other is a method of using a capillary-pumped system.
FIG. 1 shows the two-phase fluid loop using conventional mechanical pumps. The two-phase fluid loop is comprised of a condenser 101 as a heat radiating means, an a evaporator 103 as heat absorbing means, a mechanical pump 105 for driving the working fluid and a regulating valve 107.
The vapor which has absorbed heat from electronics and evaporated in the evaporator 103 as a heat absorbing means, is transported to the condenser 101 as a heat radiating means via a conduit, where heat is radiated and it is condensed. The liquid thus condensed is returned by the mechanical pump 105.
The two-phase fluid loop thus constructed has a capability for transporting a large heat load over a long distance with a small pump. In addition, the number of rotations or ratational speed of the pump and the operation of the regulating valve can be actively controlled as well as being able to deal with large thermal fluctuations.
On the other hand, however, in the flow where vapor phase and liquid phase coexist, various instabilities of the flow as well as oscillating phenomena will tend to occur. For instance, in the evaporator duct, there will occur a drooping type unstable condition wherein the flow rate becomes unstable in a negative zone of pressure loss against the increase in the flow rate and other different unstable flowing phenomena.
In the condenser as well, in the portion where vapor flows into super-cooling water, for instance, oscillations involving the condensation of vapor in liquid will tend to occur. Therefore, it is extremely difficult to carry out a stable control of the basically unstable two-phase fluid loop. Moreover, since the characteristics of the two-phase fluid are not clearly known in a zero-gravity state, the control of the two-phase fluid loop becomes a difficult problem. There are also problems of cavitation of a working fluid, lubrication of motors and reliability.
For the purpose of overcoming these problems, a capillary pumped system has been proposed heretofore, which is the two-phase fluid loop in which the amount of the evaporated liquid is automatically supplemented by capillary pumping power similar to heat-pipes.
FIG. 2 shows, by way of example, a capillary pumped heat transfer loop system according to the prior art. The loop system comprises a capillary structure 201a, that is, a wick provided at an evaporator 201 as a heat absorbing means and covering the inner walls, so as to pull in the working liquid contained in the condenser duct 203 as a heat radiating means by a capillary force.
The capillary pumped heat transfer loop system thus constructed, eliminates the necessity of an externally supplied pumping power for driving the working fluid as well as automatically supplying the amound of fluid corresponding to the evaporated amount thereof. In this capillary pumped heat transfer loop system, however, since the working fluid driving power resorts only to the capillary pumping, the fluid transport capacity is small and there is a problem in that it is difficult to transport large amounts of heat over long distances.
Moreover, the capillary structure of the heat-pipe according to the prior art is comprised of grooves and metal meshes and it has two functions; one is generation of the capillary force or capillary action so as to transport the condensate to an evaporator and the other is passage of the liquid to the condenser by the capillary force. In addition, the gap dimension of the capillary structure is perferably made small because of the generation of the capillary force while it is preferable to make the liquid channel large, so that these two points constitute an antipathetic relationship with each other. Accordingly, the gap dimension of the capillary structure is designed so as to balance the relationship. For this reason, the conventional heat pipe cannot increase the maximum flow rate of the condensate too much, thus limiting the heat transport capillary.
On the other hand, a capillary pumped system and monogroove heat-pipes which are a kind of an arterial heat-pipe have been proposed wherein only a capillary force generation function is provided in the capillary structure while a passage for transporting condensate is provided separately. These capillary pumped systems and monogroove heat pipes have some merits that external pumping power for circulating the working fluid can be omitted as well as having a self control function that the same amount of liquid as evaporated is supplied. In this system, however, since the fluid driving power resorts to the capillary force, the liquid transport capillary cannot be increased so large that it is not sufficient to transport a large amount of heat over a long distance.
As described in the foregoing, maximum flow rate of condensate cannot be increased in the heat-pipe according to the prior art and the heat transport capillary is limited. In addition, in the system wherein the amount of liquid in the evaporator is detected by an ultrasonic sensor, a valve for supplying the liquid is opened in accordance with the detected signal, and the liquid is supplied by the drive of the pump, a control mechanism and the system as a whole become complex while reliability of endurance becomes lowered.
In the mechanically pumped heat-pipe system, however, the so-called "dry out" phenomenon is produced in which the same amount of liquid corresponding to the evaporated amount of the liquid in the evaporator is not supplied to the evaporator, while when an amount of the liquid more than the evaporated amount thereof is supplied, an excessive liquid state occurs, thus lowering the heat transport efficiency for both cases. Consequently, an accurate liquid amount has to be supplied to the evaporator and how to control the liquid supply amount becomes an important problem.
As one approach for suitably controlling the liquid supply to the evaporator, it has been proposed that the residual liquid amount in the liquid channel of the evaporator is detected by an ultrasonic sensor, a liquid supply valve is opened in response to a detected signal from the sensor, and the liquid is supplied by a pump (see, for instance, "Design and Test of a Two-Phase Monogroove Cold Plate", AIAA 20th Thermophysics Conference, June 19-21, 1985).
In this structure according to the prior art, however, there are the following problems; when a flow rate in the liquid channel of the evaporator is decreased at a predetermined flow rate, an ultrasonic sensor detects this condition and transmits a command for the supply of liquid. Because of this construction, the starting and stopping of the pump as well as the opening and closing of the valve have to be intermittently and frequently carried out.
For these reasons, its control mechanism and system become complex and the reliability and endurance become lowered. In addition when the liquid amount in the liquid channel of the evaporator is decreased, there will occur discontinuity of the liquid between the vapor phase and liquid phase, with the result that a "dry out" phenomenon will occur on the evaporation surface. In order to prevent this phenomenon, a capillary structure which permits the liquid channel to communicate with the vapor surface other than the capillary structure provided on the evaporation surface.